The invention is in the field of signal coding and bandwidth management in communications. The invention can be used for coding data transmitted in any channel such as electrical, wireless, optical, cable, semiconductor waveguides, or free space media using any type of transmission channels.
In the transmission of information across a communications channel, it is generally necessary to convert the user (or source) information to the form of a signal which is compatible with transmission characteristics of the communications channel. Such conversion is in many cases accomplished through modulation of an electrical or optical carrier signal by the information content itselfxe2x80x94that electrical or optical signal being transmitted over the communications channel. In the case of information in digital form (hereafter generally referred to as xe2x80x9cdataxe2x80x9d), the xe2x80x9c1xe2x80x9ds and xe2x80x9c0xe2x80x9ds representing the data are encoded into a form that can be used to drive, or modulate an aspect of the transmission carrier signal, such as amplitude, frequency, phase, or polarity. The complexity of the modulation method varies. More bandwidth and power are required for certain modulation methods.
Various modulation techniques for digital data are known in the art, but among the more useful methods is that of xe2x80x9cline codingxe2x80x9d the user data, where the line coded data becomes the baseband signal modulating the electrical or optical channel carrier. Typically, such line codes map the data value xe2x80x9c1xe2x80x9d to a high signal value defined by the coding format and the value xe2x80x9c0xe2x80x9d to a low signal value. While, these line coding formats can include negative (less than zero) signal values for the low signal state, such a negative low signal state cannot be realized for optical transmission systemsxe2x80x94the concept of xe2x80x9cnegativexe2x80x9d light not being realizable, at least in practice. Accordingly, as the use of optical transmission media has become increasingly the norm, xe2x80x9cunipolarxe2x80x9d line codes have been developed in which the low signal state is maintained at zero or a small non-zero optical signal level.
Two line coding methods of particular interest are designated xe2x80x9cunipolar return-to-zeroxe2x80x9d (URZ) and xe2x80x9cunipolar non-return-to-zeroxe2x80x9d (UNRZ) coding formats. URZ and UNRZ coding are types of pulse/no-pulse (or, on-off-keying) modulation coding formats well known in the communications arts and are common coding methods for transmitting data in optical communications systems.
In URZ coding, the signal level during the first half of a bit interval is low for a xe2x80x9c0xe2x80x9d and high for a xe2x80x9c1xe2x80x9d. During the second half of the bit interval, the signal level returns to zero for a xe2x80x9c1xe2x80x9d and remains at zero for a xe2x80x9c0xe2x80x9d. In contrast, for UNRZ coding, the signal level is low for a xe2x80x9c0xe2x80x9d and high for a xe2x80x9c1xe2x80x9d during the full bit interval. And, as the name suggests, the signal level does not return to zero between successive xe2x80x9c1xe2x80x9ds. Both coding methods have well known advantages and disadvantages.
The main advantage of URZ is the presence of a discrete spectral component at the symbol rate, which allows simple timing recovery. Because of pulse transitions in URZ coding, it is considerably easier to recover clock information at the receiver end.
On the other hand, because of its relatively narrow pulses, more power and bandwidth are required for transmitting URZ-coded signals. In fact, URZ coding requires twice the bandwidth that UNRZ does. URZ coding consumes more power per bit than UNRZ does during transmission because of the half-bit-pulse waveform of URZ and its higher DC power content. And, if the coding requires more bandwidth, then noise power increases. As a result, URZ coding degrades signal-to-noise ratio (SNR) at the receiver end as the noise bandwidth is doubled.
For the case of UNRZ coding, a particularly important advantage, relative to URZ, is that of a relatively low bandwidth requirement. UNRZ coding also has a superior signal-to-noise ratio (SNR) to URZ.
However, UNRZ coding has its disadvantages as well. Clock recovery is more complicated with UNRZ coding as there are no clock frequency components in the signal spectrum. With UNRZ coding, for example, a loss of synchronization and timing jitter can result from the transmission of a long sequence of xe2x80x9c1xe2x80x9ds because, unlike URZ coding, no pulse transition is present. Particularly for AC-coupled systems, UNRZ leads to significant baseline wander for waveforms representing long strings of xe2x80x9c1xe2x80x9ds, while URZ has minimal baseline wander. Both URZ and UNRZ coding are susceptible to baseline wander for long strings of xe2x80x9c0xe2x80x9ds. Thus, the communications system designer is inevitably faced with certain tradeoffs in making a choice between UNRZ and URZ coding.
Both UNRZ and URZ coding have particular application in optical communications systems because of their unipolar characteristics. Since laser power is either zero or a certain positive quantity, only unipolar encoding can be implemented in fiber-optical communication systems. UNRZ is ideal for laser modulation in optical communications and is the most popular coding method in fiber-optical communication systems. URZ is also used, particularly for ultra-high-speed pulse transmission and for soliton systems, because of its narrow pulses. URZ also significantly improves data transmission performance because it makes two pulse transitions during a bit interval, whereas UNRZ-coding signals suffers loss of timing or signal-level drift during a long string of 0 or 1 bits. However, the tradeoff for choosing UNRZ coding over URZ is that URZ requires more system bandwidth.
There is therefore a need in the art of communications transmission systems, both electrical and optical, for a coding method that uses signal power as efficiently as possible, while preserving capability for clock recovery. A coding method that does not demand more system and channel bandwidth is needed, since bandwidth is expensive and increases noise power. Furthermore, a coding format is needed to avoid baseline wander so that costly restoration techniques, such as decision feedback or scrambling/descrambling mechanisms, are no longer needed. A coding method that optimally eliminates the dispersion effect of inter-symbol interference (ISI) is needed without requiring higher data rates for sampling. In short, a new coding format is needed in the art that combines the advantages of URZ and UNRZ coding, while at the same time avoiding some of the disadvantages thereof.
The invention is a unique method of generating UNRZ data from URZ data that combines the advantages of both coding methods while largely avoiding the disadvantages thereof. The coding in the invention transmits URZ data at the transmitter, along with a delayed copy thereof and receives UNRZ data by combining the two URZ pulse streams at the receiver. The invention optimally eliminates ISI due to dispersion occurring with the use of UNRZ coding alone in data communication systems. The invention also reduces bandwidth (noise) penalty incurred by transmitting and receiving URZ pulses. In addition, the invention largely avoids baseline wander so that costly (both in monetary and data rate terms) restoration techniques, such as decision feedback or scrambling/descrambling mechanisms, are no longer needed. The net bandwidth requirement of a communications system implemented according to the method of the invention, with URZ and URZd data transmitted via a transmission medium and converted to UNRZ coding at a receiver location, is the same as that of a system that transmits and receives UNRZ data.